Because he's still a living president.
Because despite younger, more online progressives with no attachment to him pretty rightfully thinking he's a creepy old dinosaur who should be dropped off at a care facility exclusively staffed by men, he's still extremely popular amongst moderate Democrats and middle aged/older Democrats. And like it or not, they still make up a huge chunk of the party.
This is the kinda thing Bernie's been getting at for a while. He's an unabashed crusader for his ideals, and that's extremely praiseworthy. But he's also a realist that, having been an outsider with his ideas for a long time, knows that he can't get his way much of the time and that there are compromises that he can make to move things in his direction. Wheeling out Clinton every 4 years for him to lay that 'ol Southern twang on us to excite the hell outta a bunch of older, more moderate folks is one of those things.
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Ted Bundy does not have the charisma Clinton does. Nor does Clinton have the rapsheet Bundy does. Nor is Bundy a living former POTUS.
A lot of people were, and I agree that's pretty fuckin gross. But like it or not, that's not a crime, and Clinton remains pretty fuckin popular amongst Democrats overall. Not the younger progressives though, who have no real connection with him.
Because the 2020 DNC is running this specifically to try to counter this GOP. That's why they're making the plays for the moderate Republicans to try to get disaffected Trump voters and moderate conservatives who think Trump is fuckin gross and live in swing states or states Trump flipped. I'm not super thrilled with the strategy as a more lefty kinda guy, but I see where they're coming from with this.